New laser detects bombs — right out of thin air

Sensing method also has uses for locating pollutants and greenhouse gases

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Researchers at Princeton University developed a technique for generating a laser beam out of nothing but air. They focus a pump laser on a distant point in the air and another laser beam comes back. The image shows a pulse of infra-red light from this.

Engineers have developed a laser-sensing technology that may allow soldiers to detect hidden bombs and scientists to better measure airborne environmental pollutants and greenhouse gases.

Researchers from Princeton University discovered the new laser sensing method, which uses an ultraviolet laser pulse that is focused on a tiny patch of air, similar to the way a magnifying glass focuses sunlight into a hot spot. Within this hot spot – a cylinder-shaped region just .04 inches long – oxygen atoms become "excited" as their electrons get pumped up to high energy levels. When the pulse ends, the electrons fall back down and emit infrared light. Some of this light travels along the length of the excited cylinder region and, as it does so, stimulates more electrons to fall, amplifying and organizing the light into a coherent laser beam aimed right back at the original laser.

"We are able to send a laser pulse out and get another pulse back from the air itself," said Richard Miles, professor of mechanical and aerospace engineering at Princeton. "The returning beam interacts with the molecules in the air and carries their fingerprints."

The researchers plan to use a sensor to receive the returning beam and determine what contaminants it encountered on the way back.

"In general, when you want to determine if there are contaminants in the air you need to collect a sample of that air and test it," Miles said. "But with remote sensing you don't need to do that. If there's a bomb buried on the road ahead of you, you'd like to detect it by sampling the surrounding air, much like bomb-sniffing dogs can do, except from far away. That way you're out of the blast zone if it explodes. It's the same thing with hazardous gases – you don't want to be there yourself. Greenhouse gases and pollutants are up in the atmosphere, so sampling is difficult."

The system most commonly used is a remote laser-sensing method, LIDAR (light detection and ranging). This method measures the scattering of a beam of light as it reflects off a distant object and returns back to a sensor. It is commonly used for measuring the density of clouds and pollution in the air, but can't determine the actual identity of the particles or gases.

The laser developed by the Princeton researchers is thousands of times stronger than LIDAR, which enables it to determine not just how many contaminants are in the air but also the identity and location of those contaminants. In addition, the new process will enable scientists to detect much smaller quantities of contaminants, which is a particular concern when trying to detect trace amounts of explosive vapors. Any chemical explosive emits various gases depending on its ingredients, but for many explosives the amount of gas is miniscule.

So far, the researchers have demonstrated the process in the laboratory over a distance of about a foot and a half. In the future they plan to increase the distance over which the beams travel, and they also plan to fine-tune the sensitivity of the technique to better identify small amounts of airborne contaminants.

"We'd like to be able to detect contaminants that are below a few parts per billion of the air molecules," Miles said. "That's an incredibly small number of molecules to find among the huge number of benign air molecules."

The researchers published their new method Jan. 28 in the journal Science.